According to the conditions of road and traffic, a driver may use a series of special devices mounted on the motor vehicle to force the road surface to apply definite external force on the motor vehicle wheels in the direction being opposite to the running direction of the motor vehicle, thus generating forced brake on the motor vehicle to certain extent. Such controllable external force for braking the motor vehicle is called as braking force, and a series of special devices used for generating the braking force are called as brake system. In terms of functions, the braking systems are classified into two types, namely the service brake system and the parking brake system. The service brake system refers to a series of devices designed to allow the running motor vehicle to speed down or even stop, acting as the major brake system for the motor vehicle. The parking brake system refers to the device designed to maintain the standstill state of the motor vehicle in parking, playing the role as the auxiliary brake system for the motor vehicle.
According to different braking energy sources, the parking brake system is also divided into two types, namely the traditional mechanical type parking brake system and the electronic parking brake system. In the application of the mechanical type parking brake system, the parking brake pull rod is manipulated by the driver, and the parking brake pull rod drives the brake drum to open up or drives the braking caliper piston to move to complete parking. In such case, the braking force is completely originated from the driver. Since different drivers may apply different forces, it is very likely that insufficient braking force may be caused, leading to the risk of slipping.
The mechanical type parking brake system may have many inconveniences in operation, manly including: at the time of starting the motor vehicle, the driver is not only required to control the steering as well as the accelerator pedal and the clutch, and is also required to release the parking brake pull rod, so that the starting operations become very complicated. Such complicated operations are particularly reflected in the starting on a slope. In such case, the driver is also required to select the opportunity to release the parking brake pull rod. If the parking brake pull rod is released too soon, the risk of slipping along the slope will be generated; if it is released too late, flame-out of the engine will be caused.
To overcome said technical problems in the traditional mechanical type parking brake system, electrical parking brake (EPB for short) system applied in the motor vehicle has been developed in the prior art.
EPB system is designed to apply the braking force through the motor. In the process of parking, the driver is merely required to press the parking button, and the controller of EPB system will control the rotate of the motor and thus complete parking. It can always apply the maximal pressing force. Furthermore, EPB system can also make communication with the other control modules of vehicle (such as engine controller, ABS/ESP controller and vehicle body controller), so as to realize the function of automatic parking/automatic release, simplify the driver's operations and ensure the safety and comfort level in driving.
However, it is difficult to judge the conditions for realizing automatic release in EPB system, because it is necessary to consider the potential safety hazards that may occur in the process of release. Neither the sliding slope resulted from premature release nor the flame-out of engine/even the damage of the transmission system due to too late automatic release is allowed. To judge whether automatic release is feasible, it is critical to know whether the power of the engine has been transmitted to the wheels and whether such power is sufficient to ensure that the sliding slope will not occur.
To solve said technical problems, Germany Lucus Automobile Co., Ltd has successively disclosed two Chinese patent applications, namely “A parking brake and a control method thereof” (Application number: 200480038712.7) and “A method for operating the braking device of vehicle” (Application number: 200580003615.9). While China Zhejiang Asia-Pacific Mechanical & Electronic Co., Ltd disclosed a Chinese patent “Control Method of Automobile Electric Control Parking and Start Assisting By Manual Transmission And System Thereof” (Application number: 200910097642.7). In the technical solution published by this patent application, it is necessary to add some sensors, such as clutch position sensor and gear position sensor (such as position switch), to realize the automatic release control of parking. Through the combined information of the clutch position transducer and the gear position sensor, it is feasible to judge whether the power of the engine has been transmitted to the wheels. However, this system involves many technical problems in practical application.
1. EPB system must depend on the gear position sensor and the clutch position transducer on the motor vehicle, otherwise it is neither impossible to obtain the clutch position and gear position information nor know whether the power has been transmitted to the wheels.
2. In order to additionally mount the clutch position sensor and the gear position sensor, it is required to redesign the clutch operating device and the gear shifting operating mechanism.
3. Increase cost.
4. Since the friction disc will gradually wear off in the application of the clutch, and its engagement point is also constantly changing, the error of the clutch position sensor will become increasingly higher. As a result, EPB system will easily make a mistake in judging the driving intention of the driver. Therefore, when the automatic parking is released, misjudgment and misoperation will still occur. Upon occurrence of misoperation, the condition of sliding slope or flameout of the engine will occur. Furthermore, the service life of the clutch will be greatly reduced over the long-term.